Israeli aircraft have killed three Palestinians in the Gaza Strip in the latest round of fighting with militants who fired rockets into Israel for a third day.

Palestinian medics say Israeli airstrikes on Sunday killed a militant, a 60-year old man and a 12-year-old boy who was walking to school in the northern Gaza town of Jebaliya.

The Israeli military says it was targeting militants who fired at least a dozen rockets into southern Israel during the day. No casualties were reported in the rocket attacks.

Israeli airstrikes on Gaza have killed 18 Palestinians since Friday, mostly militants affiliated with Islamic Jihad and the Popular Resistance Committees. The two groups have fired more than 100 rockets into Israel in that period, wounding several civilians. Israel's Iron Dome anti-missile system has intercepted about 30 of the rockets.

It is one of the deadliest exchanges of fire between Israel and Gaza militants since they ended a brief war in January 2009.

Israeli Prime Minister Benjamin Netanyahu said Sunday the airstrikes have exacted a "heavy price" from the militants and will continue as long as necessary. Israeli authorities have shut schools in the country's south in response to the rocket fire.

Palestinian officials say Egypt is trying mediate a return to calm in the area, as it has done during previous, smaller-scale exchanges of fire between the two sides in recent years.

An Islamic Jihad leader, Khaled Batch, told the Associated Press there is no room for calm while "Zionist aggression" against Gaza continues. Gaza has been run by the larger Hamas militant group since 2007.

The latest fighting erupted Friday when an Israeli air strike on Gaza killed two PRC leaders, triggering retaliatory rocket fire. Israel said one of the senior militants, Zuhair al-Qaissi, planned an attack last August in which militants infiltrated southern Israel from Egypt and killed eight Israelis. Israeli officials said al-Qaissi was plotting to carry out a similar attack in the near future.

Thousands of Palestinian mourners took to the streets of Gaza on Saturday for funeral processions of those killed in the Israeli airstrikes. Many of the marchers chanted "Revenge, revenge!"

The United States has condemned the rocket attacks on Israel as "cowardly acts" and appealed to both sides to restore calm.